Many pelagic fishes swim through the water column in a series of continuous, vertical 'yo-yo' dives. Possible functions of these vertical movements include behavioral thermoregulation, foraging, navigation and energy conservation. Pelagic sharks show similar vertical movement, and also tend to be negatively buoyant, but the role of these dives in sharks, and the mechanics behind them are unclear. Tri-axial accelerometer data loggers have revolutionized studies of animal behavior and are providing unprecedented new insight into the biomechanics of shark swimming. These devices measure accelerations due to gravity in 3-dimensions, and can be used to reconstruct a high-resolution record of shark body movements, including tail beat frequency and amplitude. This provides an opportunity to address long-standing questions such as whether sharks ‘fly-glide’ to save energy. In theory, negatively buoyant fish could reduce the overall cost of locomotion by nearly 50% by using powerless gliding during descent and actively swimming during ascent but this behavior has never been empirically demonstrated in sharks.

Research Questions

We asked three specific questions about tiger shark swimming behavior:

Figure 2. A released tiger shark swims away equipped with the datalogger package.

Results

Tiger sharks swam continuously with frequent vertical movements within the mixed layer at mean swim speeds of 0.5-0.9 m s-1, and showed burst swim events during both the descent and ascent phases of the dives. Tail beat frequency was affected by both swimming speed and whether the shark was ascending or descending. Sharks swam with a higher tail beat frequency while ascending than descending, and only occasionally glided for short periods while descending (<3% of total descent time), indicating that their negative buoyancy affected swimming behaviors. Camera images showed many potential prey items (fishes).

Figure 3. In this image from the shark-mounted camera, a school of reef fishes swims straight down toward the reef as the tiger shark approaches.

Ongoing Shark Accelerometer Research

We are building on our successful short-term accelerometer deployments. Our future goals are (1) to increase the accelerometer deployment lengths to capture longer time series of shark behavior, and (2) To instrument other shark species with these devices. We have already successfully obtained 48h accelerometer tracks from a large tiger shark, and 24h deployments on Galapagos sharks (Carcharhinus galapagensis).